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August 2017 www.laserfocusworld.com Laser Focus World 8frequency of the resonator. The new la-sers are now being used both at PTB andat JILA to further improve the quality ofoptical atomic clocks and to carry out newprecision measurements on ultracold at-oms. At PTB, the ultrastable light fromthese lasers is already being distributedvia optical waveguides and is then usedby the optical clocks in Braunschweig. Thescientists from this collaboration see fur-ther optimization possibilities. With novelcrystalline mirror layers and lower temper-atures, the disturbing thermal noise canbe further reduced. The linewidth couldthen even become smaller than 1 mHz.

Graphene-basedWi-Fi antenna forwindows is opticallytransparent

An optically transparent half-wavelength microwave antenna made
by layering monolayer graphene onto
glass has been created by a group at
Aoyama Gakuin University (Kanagawa,
Japan). The antenna could be used, for
example, as a transparent wireless-communications antenna that would
simply be part of an architectural
window, an automotive window,
eyeglasses, or other glass surface.
While indium tin oxide (ITO)-based
transparent microwave windows
have previously been fabricated, the
researchers say that their graphene
version removes the need to use indium,
which is a rare-earth metal.

The antenna itself is actually rather
small: 10. 7 × 1.0 mm. It includes a
gold-film coplanar waveguide (CPW)
at the edge of the window connected
by a gold-film transmission line to
the graphene antenna itself. To
create the device, graphene was
first grown on a copper foil via low-pressure chemical-vapor deposition—
the graphene was then transferred
to the experimental 1-mm-thick, 20

× 20 mm square synthetic quartz
substrate. The shape of the antenna
and CPW were then patterned using
standard photolithographic techniques.
Monolayer graphene has a visible
absorption of only around 2%, so is
highly transparent. Measured reflection
minima at the operational microwave
frequencies, as well as the detection
of the 20. 7 GHz operating microwave
radiation, showed that the antenna was
working well, with about 90% of the
microwave power reaching the antenna
elements. The researchers are now
working on integrating the antenna into
optoelectronic devices. Reference: S.
Kosuga et al., Appl. Phys. Lett. (2017);
http://dx.doi.org/10.1063/1.4984956.